KHUN SAMUT CHIN, Thailand - At Bangkok's watery gates, Buddhist monks cling to a shrinking spit of land around their temple as they wage war against the relentlessly rising sea.

During the monsoons at high tide, waves hurdle the breakwater of concrete pillars and the inner rock wall around the temple on a promontory in the Gulf of Thailand. Jutting above the water line just ahead are remnants of a village that has already slipped beneath the sea.

Experts say these waters, aided by sinking land, threaten to submerge Thailand's sprawling capital of more than 10 million people within this century. Bangkok is one of 13 of the world's largest 20 cities at risk of being swamped as sea levels rise in coming decades, according to warnings at the recent Intergovernmental Panel on Climate Change held here.

"This is what the future will look like in many places around the world," said Lisa Schipper, an American researcher on global warming, while visiting the temple. "Here is a living study in environmental change."

The loss of Bangkok would destroy the country's economic engine and a major hub for regional tourism.

"If the heart of Thailand is under water, everything will stop," says Smith Dharmasaroja, chairman of the government's Committee of National Disaster Warning Administration. "We don't have time to move our capital in the next 15-20 years. We have to protect our heart now, and it's almost too late."

The arithmetic gives Bangkok little cause for optimism. The expanding megalopolis rests about 31/2 to 5 feet above the nearby gulf, although some areas already lie below sea level. The gulf's waters have been rising by about a tenth of an inch a year, about the same as the world average, said Anond Snidvongs, a leading scientist in the field.

But the city, built on clay rather than bedrock, has also been sinking at a far faster pace of up to 4 inches annually as its teeming population and factories pump about 2.5 million cubic tons of cheaply priced water, legally and illegally, out of its aquifers. This compacts the layers of clay and causes the land to sink.

The government, scientists and environmental groups agree Bangkok is headed for trouble, but there is some debate about when. Anond, who heads the Southeast Asia START Regional Center, believes total submersion may not be imminent; Smith disagrees.

"You notice that every highway, road and building which has no foundation pilings is sinking," Smith said. "We feel that with the ground sinking and the seawater rising, Bangkok will be under seawater in the next 15 to 20 years - permanently."

Once known as the "Venice of the East," Bangkok was founded 225 years ago on a swampy floodplain along the Chao Phraya River. But beginning in the 1950s, on the advice of international development agencies, most of the canals were filled in to make roads and combat malaria. That fractured the natural drainage system that had helped control Bangkok's annual monsoon-season flooding.

Smith urges that work start now on a dike system of more than 60 miles - protective walls about 16 feet high, punctured by water gates and with roads on top, not unlike the dikes long used in the low-lying Netherlands to ward off the sea. The dikes would run on both banks of the Chao Phraya River, then fork to the right and left at the mouth of the river.

Anond, an oceanographer who studied at the University of Hawaii, says other options must be explored. "There is no one single solution to respond to climate change," he said.